To state the obvious...
With either speaker, 5" or 6", I'd say a sub is mandatory. don't expect that extra inch to make that much difference.
And, while the Onix might be "decent", if you can find a Dayton sub, either assembled or in kit form, for around $100 - $150 range, I'd strongly recommend it. Their SUB-80, SUB-100 and SUB-120 come to mind.